This invention relates to seals and more particularly to seals for components of closure heads of nuclear reactors.
In nuclear reactor designs, well known in the art, a reactor vessel with fuel assemblies disposed therein, and having an inlet and an outlet for circulation of a coolant in heat transfer relationship with the fuel assemblies, is sealed by a closure head located on top of the reactor vessel. In certain designs, the closure head comprises one or more rotatable plugs. These rotatable plugs, which may be of varying sizes disposed eccentrically within each other, serve at least two purposes. One purpose is, of course, to seal the reactor internals inside the reactor vessel. Another purpose is to support refueling machines. The rotation of the rotatable plugs positions the refueling machines in appropriate relationship to the fuel assemblies in the reactor vessel to facilitate the refueling process. Since the rotatable plugs must be able to rotate relative to each other, the plugs are mounted so as to define an annulus between them. The annulus, while allowing the rotation of the plugs, also establishes a path for the release of radioactive particles located in the reactor vessel. Accordingly, seals are provided at various locations across the annulus to prevent this release of radioactive particles. The seals also function to prevent oxygen in the atmosphere outside the reactor vessel from passing through the annulus to the reactor coolant which in a liquid metal fast breeder reactor may be liquid sodium because contact of liquid sodium with oxygen may result in a violent reaction. To further prevent oxygen leakage into the reactor vessel, a cover gas is provided that fills the space from the top of the reactor coolant pool to the bottom of the closure head and up the annulus to the seals across the annulus.
In the process of designing such liquid metal fast breeder reactors, it is common practice to analyze the effectiveness of the closure head seals under extreme conditions that are highly unlikely to occur to thereby assure the effectiveness of such seals under normal conditions. During the course of such design the seals are subjected to a sophisticated analysis which determines the seal response under a hypothetical core disruptive accident (CDA), theoretically the worst possible accident. Typically, the CDA is hypothesized to be a case in which, for whatever reason, a void violently propagates in the reactor coolant causing a violent expansion of the reactor coolant which in turn forces the cover gas up the annulus between the closure head plugs where the cover gas and liquid sodium are hypothesized to impact the closure head seals which are across the annulus. To meet design requirements, the seals must be able to contain the cover gas and liquid sodium which will have radioactive particles therein in order to prevent a release of radioactive particles to outside the reactor vessel. There are several kinds of effective closure head seals known in the art.
One type of closure head seal well known to those skilled in the art is a liquid dip seal. In a liquid dip seal, the annulus between the closure head plugs is contoured so that a trough is formed by the annulus itself. A liquid such as liquid sodium is placed in the trough thereby dividing the annulus into two sections, one above the liquid and one below. The cover gas, inside the reactor, containing radioactive particles then extends from the top of the reactor coolant pool up through the annulus to the liquid sodium in the dip seal. The liquid dip seal under normal conditions provides an effective seal against cover gas migration out of the annulus and against oxygen migration into the reactor vessel while allowing the rotatable closure head plugs to rotate relative to each other. However, under the CDA analysis, the expansion of the reactor coolant could force the cover gas up the annulus in a violent manner. In the process, the cover gas could expel the liquid sodium from the dip seal onto seals and bearings located in the annulus above the dip seal, thereby rendering the liquid dip seal ineffective under such hypothesized conditions.
Another type of closure head seal well known in the art is the inflatable seal wherein single or multiple inflatable seals in series are placed across the annulus in a manner well known in the art. During reactor refueling, the inflatable seals are slightly deflated to allow better rotation of the rotatable closure head plugs while during reactor operation the seals are inflated to increase their sealing capability. While inflatable seals are effective under normal conditions, their effectiveness under a CDA analysis is sometimes questioned.
Still another seal well known in the art and designed specifically for CDA conditions is a type of labyrinth seal in which a piece of metal is bolted to one of the closure head plugs so as to extend across the annulus between the plugs to within close proximity to the other plug. The purpose of this seal is to effectively lower the leak path area to thus limit leakage under CDA. However, when subjected to analysis, this seal while theoretically reducing leakage and allowing plug rotation, does not completely solve the problem of preventing release of radioactive particles under severe reactor conditions such as CDA.
In addition, the other commonly known types of seals such as O-rings, bellows, etc., while possibly effective under CDA conditions, do not allow for effective rotation of the closure head plugs.